1. Field of the Invention
The present invention relates to RF plasma processing reactors and, more particularly, to an inventive plasma reactor which uses a radio frequency (RF) energy source for electromagnetically coupling the associated RF electromagnetic wave to the plasma, and a silicon source in contact with the plasma, and to processes carried out in the reactor.
2. Description of Related Technologies
The trend toward increasingly dense integrated geometries has resulted in components and devices of very small geometry which are electrically sensitive and susceptible to damage when subjected to wafer sheath voltages as small as approximately 200-300 volts due to energetic particle bombardment or radiation. Unfortunately, such voltages are of smaller magnitude than the voltages to which the circuit components are subjected during standard integrated circuit fabrication processes.
Structures such as MOS capacitors and transistors fabricated for advanced devices have very thin (thickness  less than 200 Angstroms) gate oxides. These devices may be damaged by charge-up, resulting in gate breakdown. This can occur in a plasma process when neutralization of surface charge does not occur, by non-uniform plasma potential/or density, or by large RF displacement currents. Conductors such as interconnect lines may be damaged for similar reasons as well. Etching processes carried out in conventional plasma etch chambers are also becoming inadequate when very high aspect ratio, i.e., very deep and very narrow openings and trenches, must be formed in, or filled with, a variety of semiconductor materials.
RF Systems
Consider first prior art semiconductor processing systems such as CVD (chemical vapor deposition) and RIE (reactive ion etching) reactor systems. These systems may use radio frequency energy at low frequencies from about 10-500 KHz up to higher frequencies of about 13.56-40.68 MHz. Below about 1 MHz, ions and electrons can be accelerated by the oscillating electric field, and by any steady state electric field developed in the plasma. At such relatively low frequencies, the electrode sheath voltage produced at the wafers typically is up to one or more kilovolt peaks, which is much higher than the damage threshold of 200-300 volts. Above several MHz, electrons are still able to follow the changing electric field. More massive ions, however, are not able to follow the changing field, but are accelerated by steady state electric fields. In this frequency range (and at practical gas pressures and power levels), steady state sheath voltages are in the range of several hundred volts to 1,000 volts or more.
Magnetic Field-Enhancement
A favorite approach for decreasing the bias voltage in RF systems involves applying a magnetic field to the plasma. This B field confines the electrons to the region near the surface of the substrate and increases the ion flux density and ion current and, thus, reduces the voltage and ion energy requirements. By way of comparison, an exemplary non-magnetic RIE process for etching silicon dioxide might use RF energy applied at 13.56 MHz, an asymmetrical system of 10-15 liters volume, 50 millitorr pressure and an anode area to wafer-support cathode area ratio of approximately (8-10) to 1, and develop substrate (cathode) sheath voltage of approximately 800 volts. The application of a magnetic field of 60 gauss may decrease the bias voltage approximately 25-30 percent, from 800 volts to about 500-600 volts, while increasing the etch rate by as much as about 50 percent.
However, the application of a stationary B field parallel to the substrate develops an Exc3x97B ion/electron drift and an associated plasma density gradient which is directed diametrically across the substrate. The plasma gradient causes non-uniform etching, deposition and other non-uniform film properties across the substrate. The non-uniformities may be decreased by rotating the magnetic field around the substrate, typically either by mechanical movement of permanent magnets, or by using pairs of electromagnetic coils which are driven in quadrature, 90 degrees out of phase, or by instantaneously controlling the current in pairs of coils to step or otherwise move the magnetic field at a controlled rate. However, although rotating the field reduces the non-uniformity gradient, typically some degree of non-uniformity remains.
Furthermore, it is difficult to pack coils and, in particular, to pack two or more pairs of coils about a chamber and to achieve a compact system, especially when using a Helmholtz coil configuration and/or a multi-chamber system of individual magnetic-enhanced reactor chambers surrounding a common loadlock.
A unique reactor system which has the capability to instantaneously and selectively alter the magnetic field strength and direction, and which is designed for use in compact multi-chamber reactor systems, is disclosed in commonly assigned U.S. Pat. No. 4,842,683, issued Jun. 27, 1989, in the name of inventors Cheng et al.
Microwave/ECR Systems
Microwave and microwave ECR (electron cylotron resonance) systems use microwave energy of frequencies  greater than 800 MHz and, typically, frequencies of 2.45 GHz to excite the plasma. This technique produces a high density plasma, but low particle energies which may be below the minimum reaction threshold energy for many processes, such as the reactive ion etching of silicon dioxide. To compensate, energy-enhancing low frequency electrical power is coupled to the substrate support electrode and through the substrate to the plasma. Thus, the probability of substrate damage is decreased relative to previous systems.
However, microwave and microwave ECR systems operated at practical power levels for semiconductor substrate processing such as etch or CVD require large waveguides for power transmission, and expensive tuners, directional couplers, circulators, and dummy loads for operation. Additionally, to satisfy the ECR condition for microwave ECR systems operated at the commercially available 2.45 GHz, a magnetic field of 875 gauss is necessary, requiring large electromagnets and large power and cooling requirements.
Microwave and microwave ECR systems are not readily scalable. Hardware is available for 2.45 GHz, because this frequency is used for microwave ovens. 915 MHz systems are also available, although at higher cost. Hardware is not readily or economically available for other frequencies. As a consequence, to scale a 5-6 in. microwave system upward to accommodate larger semiconductor substrates requires the use of higher modes of operation. This scaling at a fixed frequency by operating at higher modes requires very tight process control to avoid so-called mode flipping to higher or lower order loads and resulting process changes. Alternatively, scaling can be accomplished, for example, for a 5-6 in. microwave cavity, by using a diverging magnetic field to spread out the plasma flux over a larger area. This method reduces effective power density and thus plasma density.
Further, ECR systems must be operated at very low pressures, on the order of 2-3 millitorr, because the density of the plasma generated in the system falls very rapidly above about 2-3 millitorr. This requires that a large volume of reactive gases be fed to the system, and also requires large vacuum exhaust systems to remove these large volume of gases.
HF Transmission Line System
Previously mentioned, commonly assigned parent patent application U.S. Ser. No. 559,947, entitled VHF/UHF REACTOR SYSTEM, filed Jul. 31, 1990, in the name of the inventors Collins et al is incorporated by reference. This incorporated application discloses a high frequency VHF/UHF reactor system in which the reactor chamber itself is configured in part as a transmission line structure for applying high frequency plasma generating energy to the chamber from a matching network. The unique integral transmission line structure permits satisfaction of the requirements of a very short transmission line between the matching network and the load and permits the use of relatively high frequencies, 50 to 800 MHz. It enables the efficient, controllable application of RF plasma generating energy to the plasma electrodes for generating commercially acceptable etch and deposition rates at relatively low ion energies and low sheath voltages. The relatively low voltage reduces the probability of damage to electrically sensitive small geometry semiconductor devices. The VHF/UHF system avoids various other prior art shortcomings, such as the above-described scalability and power limitations.
In one aspect, our invention, which overcomes prior art shortcomings, is embodied in the construction and operation of an RF plasma processing system comprising a vacuum chamber having a source region; a substrate support electrode in the chamber; a source of process gas in the chamber; means for electromagnetically coupling RF energy into the source region for generating a plasma from said process gas; means for capacitively coupling RF energy to the substrate support electrode to control the plasma sheath voltage at the support electrode and a source of silicon in contact with the plasma.
Suitably an RF antenna means is adjacent the source region and is coupled to the RF energy source. Preferably the chamber of the invention has a triode arrangement comprising an RF cathode in the processing region, an anode defined by-the chamber walls, and a third electrode which can be electrically floating, grounded or connected to RF bias, for enhancing plasma processing. The third electrode and/or the chamber walls defining the source region, may be the source of silicon for enhancing processes such as oxide etching. Using the apparatus of the invention, plasma etching can be made almost infinitely selective, and plasma deposition made void free.